Actuators for optical systems are typically used to reposition one or more lenses of the optical system with respect to an image plane to alter focal length of the optical system. The repositioning generally is intended to achieve either focus or zoom. Actuators for achieving focus are used to adjust the focal length of the optical system in order to make an image distinct or clear. For small scale optical systems, especially in hand-held devices such as phones, electromagnetic actuation systems (aka voice coil motors) systems have been used for focusing. In such arrangements, lenses typically move less than 350 μm along the optical axis to focus. Such electromagnetic systems apply current to the actuator to effect movement of the optical components in a single direction along the optical axis. This movement is counteracted by a spring, which pulls the optical components in the opposite direction. The distance moved is thus a function of the applied current vis-à-vis the tension of the spring.
Actuators for achieving zoom reposition one or more lenses of the optical system with respect to the image plane so the focal length of the optical system is varied to cause a distant object to appear closer without moving the camera. For example, a 2:1 zoom lens at maximum focal length can make an object appear only half as distant from the image sensor as it appears with the zoom lens at minimum focal length.
The proliferation of small scale optical systems for use in, for example, a variety of miniature devices, such as cellphones, tablets, and surveillance cameras, places additional challenges on actuation systems due to a small form factor yet still requiring good performance. The desired performance characteristics of actuator systems in small scale optical systems include positional accuracy, low power, low noise levels, and speed. Positional accuracy is important for achieving desired image quality. Low power consumption is important for prolonging battery life in a variety of handheld mobile devices, and is affected by required stroke length, the force needed to overcome the weight of the moving optical components, and friction. Avoiding or minimizing acoustic noise generated during actuation is important to prevent the capturing of undesired noise by device microphones during video capture. Speed in achieving composition and focus of the desired image (including zoom), which is a function of the speed of component movement as well as displacement distance, is important for meeting the consumers' desired response time for either a zoomed-in or a focused image.
In many modern optical systems, zoom can also be achieved through software means, typically referred to as “digital zoom.” Digital zoom is a method of decreasing (narrowing) the apparent angle of view of a digital photographic or video image. Digital zoom is accomplished by cropping an image down to a centered area with the same aspect ratio as the original. Digital zoom is accomplished electronically, with no adjustment of the camera's optics, and no optical resolution is gained in the process. The cropping leads to a reduction in the quality of the image. In many instances, digital zoom also includes interpolating the result back up to the pixel dimensions of the original. This combination of cropping and enlargement of the pixels typically creates a pixelation/mosaic effect in the image, and typically introduces interpolation artifacts. Such pixelation typically results in an image of significantly reduced quality. In addition, digital zoom has typically been implemented as a series of increments, rather than continuous zoom. Thus, for example, some digital zooms are implemented in one-tenth power increments, while others use larger increments. This corresponds to a reduction in the effective size of the sensor.
Some prior art has attempted to overcome the shortcomings of digital zoom by providing an oversized sensor, for example forty-one megapixels. In such an arrangement, the narrowing of the field of view, and thus the cropping, that is inherent in digital zoom still illuminates a significant number of megapixels. The resulting, zoomed image therefore appears more acceptable even though cropped in the conventional manner. In one version of such a prior art design, the full size sensor is said to be used at the full field of view (or widest angle), but an image at 2× zoom uses only approximately eight megapixels of that sensor, and an image at 3× zoom uses only about five megapixels of the sensor.
However, the oversized sensor is physically larger than is desirable for mobile devices such as cellular phones, in addition to being prohibitively expensive for most such devices. In addition, the larger sensor also needs a longer optical path to the sensor, to ensure the image covers the entire sensor. As such, there is still often a z-axis protrusion, not just a larger x, y of the sensor.
Unlike digital zoom, optical zoom has long been used in photography and other optical systems to provide zoom without loss of image quality. Typical lens systems which provide optical zoom using concave or convex lens elements move one or more lens elements along the optical axis, and in most such systems the optical center of each lens element is located on the optical axis. While such systems can provide excellent image clarity, they require that the lens elements travel too great a distance to be suitable for many applications which require a small form factor. For example, in cameras used in cellular phones, the electronics of the cellular phone imposes severe limits on the form factor of the lens module used in the cell phone's camera, and such limits prohibit the use of conventional optical zoom.
While some cellular phones have offered lens systems which provide optical zoom for use with their integrated cameras, these have typically greatly increased the thickness of the cell phone in at least the area of the camera's lens system. In addition, prior art optical zoom capable of, for example, 3× magnification, where the lenses move along the optical axis, typically require the optical components to move greater than 10 mm. Such a long travel range typically requires the use of stepper motors. This is not ideal for small scale optical systems used in mobile devices as they are bulky, require more power to move such a long distance, and may cause acoustic noise to be picked up by device's microphone. Other actuation systems include piezo motors to actuate optical components parallel to the optical axis to create optical zoom and autofocus. However, piezo motors also tend to be acoustically noisy as well as creating hysteresis issues requiring more complicated electronics to overcome. Piezo motors also tend to use more power than some other designs. In addition, systems using conventional concave-convex lenses to offer optical zoom require a significantly longer stroke than is currently desired, increasing battery drain as well as requiring a significantly larger form factor for the lens module.
There has therefore been a long felt need for an optical system suitable for use in mobile devices such as cellular phones or other small scale systems which provides the clarity of optical zoom in a small form factor, yet does not require excessive power.